We propose to synthesize five representative members of the biologically important class of naturally-occurring polyethers, including the antibiotics septamycin, nigericin, lasalocid A, and ionomycin, and the algal metabolite thyrsiferol. Versatile stereoselective methods for producing substituted tetrahydrofurans and tetrahydropyrans by cyclization of olefinic alcohol derivatives with relative asymmetric induction will be developed. These methods involve electrophilic cyclization of ether derivatives and hydroperioxides (for the selective formation of cis-2,5-disubstituted tetrahydrofurans) and cyclization of the alcohols themselves or ring contraction of tetrahydropyrans (for the selective formation of the trans-isomers).